This invention relates to airfoils and more particularly, to an erosion resistant nanocomposite layered airfoil.
As airfoils, such as propellers, helicopter rotor blades, and gas turbine blades translate, they pass through the air and create certain directional forces. For example, as a helicopter rotor blade rotates through the air, the blade creates lift, thereby elevating the helicopter. The air through which the helicopter rotor blade rotates, however, may contain particulate matter, such as sand. The size of sand typically ranges from about 0.1 to 2000 microns and more typically from about 20 to 30 microns. If the air contains sand, the sand impinges upon the airfoil as it rotates, thereby causing abrasion to the airfoil, or portions thereof. Unless the airfoil is adequately protected, such repetitive abrasive contact could eventually cause the airfoil to erode.
The potential for erosion also exists if the airfoil circulates through air containing particulate matter such as water droplets (i.e., rain). The size of water droplets ranges from about 1000 to 4000 microns and is typically about 2000 microns. Although the size of the water droplets is typically greater than the size of sand, under high velocity conditions, such as those in this instance, water droplets behave similarly to sand, thereby causing erosion. Moreover, the combination of rain and sand exacerbates the amount of abrasion caused by such particulate matter. Therefore, when translating an airfoil through air comprising both rain and sand, the potential for erosion further increases.
The potential for erosion is also a function of the force at which the particulate matter impacts the airfoil. Specifically, as the impact force increases, so does the potential for erosion. The force at which the particulate matter impacts the airfoil is dependent upon the geometric shapes of both the airfoil and the impacting particle and their relative velocities. Particularly, the leading edge of the airfoil is the portion of the airfoil that first cleaves through the air. Therefore, the leading edge is the portion of the airfoil most susceptible to erosion caused by the particulate matter""s abrasive contact.
The amount of erosion to the airfoil is also a function of the velocity at which the particulate matter impacts the leading edge. In other words, the potential for erosion increases as the rotational speed of the airfoil increases. Increasing the rotational speed of the airfoil, however, may be necessary to produce the desired lift or power. Hence, the desire for increased power or lift is counterproductive to erosion prevention. Because an airfoil typically rotates around a central axis, the velocity of the airfoil, relative to the air, differs along the leading edge of the airfoil. More specifically, the velocity at a point on an airfoil is equal to the product of the distance from the center rotational axis and the rotational rate of such airfoil. As the distance from the rotational axis along the leading edge of the airfoil increases, so does the velocity at a given point on the airfoil. The outboard tip of the airfoil is the furthest from the rotational axis. Therefore, the potential for erosion is greatest at the outboard tip of the leading edge of the airfoil.
Various techniques attempting to minimize the amount of erosion to the leading edge of airfoils currently exist. One such technique includes adhesively bonding an appropriately shaped piece of ductile metal onto the leading edge of the airfoil, such that the ductile metal is an integral part of the airfoil. The ductile metal leading edge is typically constructed of nickel, which provides increased wear resistance. The nickel""s extended exposure to impinging particulate matter, however, causes the ductile metal leading edge to erode. The eroded nickel must, therefore, be replaced. Because the ductile metal leading edge is adhesively bonded to the airfoil, replacing the ductile metal leading edge requires a certain amount of time and skill, which is not typically available within the field. Repairs that are performed in the field are referred to as xe2x80x9cfield levelxe2x80x9d repairs because such repairs require an acceptable amount of time and a minimal amount of skill to complete. Repairs requiring an extended amount of time and a heightened skill level occur back at the aircraft depot and are referred to as xe2x80x9cdepotxe2x80x9d repairs. Depot repairs are undesirable because depot repairs increase the amount of time that the aircraft is unavailable in comparison to a field level repair. Because the replacement of the ductile metal leading edge is considered a depot repair, bonding ductile metal onto the leading edge of an airfoil is an undesirable technique for minimizing erosion.
One type of xe2x80x9cfield levelxe2x80x9d repair technique for improving an airfoil""s wear resistance includes applying an elastomeric material to the leading edge of the airfoil. Typically, the elastomeric material is applied to the leading edge as a tape. As the tape becomes worn, it can quickly and easily be removed, and a new layer of tape can be applied. Unfortunately, the frequency at which the elastomeric tape must be replaced is high compared to the rate at which the metal leading edge must be replaced because the elastomer""s ability to resist erosion, caused by the combined rain and sand, is less than that of nickel. Specifically, the elastomeric tape fails to adequately absorb the impact energy of the particulate matter. Without adequate absorption capabilities, the elastomer fails to dissipate the impact energy, thereby allowing the particulate matter to tear (i.e., destroy) the elastomer. Without frequent replacement of the elastomeric tape, the leading edge of the airfoil remains unprotected. The elastomeric tape, therefore, fails to adequately protect the airfoil from erosion for any substantial amount of time.
An alternate technique for increasing the airfoil""s wear resistance includes producing an engineered ceramic component, as discussed in U.S. Pat. No. 5,542,820, which is hereby incorporated by reference. That patent describes an engineered ceramic component having an aerodynamic ceramic member encapsulating a ductile composite infrastructure with a strain isolator member therebetween. The strain isolator member provides strain attenuation between the aerodynamic ceramic member and the ductile composite infrastructure. The engineered ceramic component is a structure comprised of five layers. Two of the five layers include adhesive bond layers. One adhesive bond layer exists between the strain isolator member and the aerodynamic ceramic member, and the other adhesive bond layer exists between the strain isolator member and the ductile composite infrastructure. The time and skill level required to replace this complex structure more closely resembles that for replacing the metal leading edges than that for replacing the elastomeric tape. Replacing the engineered ceramic component, therefore, is more closely related to a depot repair than a field repair. Although it is possible to replace the multi-layered structure in the field, doing so is more complex than replacing the elastomeric tape. Furthermore, the cost of an engineered ceramic component makes this an expensive repair.
What is needed is an inexpensive, uncomplicated xe2x80x9cfield levelxe2x80x9d repair technique that provides similar benefits as the engineered ceramic component, thereby increasing the airfoil""s wear resistance, without the need to frequently repeat the process.
The present invention is a nanocomposite layer applied to the leading edge of an airfoil. The nanocomposite is an elastomeric matrix reinforced with nanosized reinforcing particles ranging in size from about 0.5 to 1000 nanometers and preferably ranging in size from about 5 to 100 nanometers. These nanosized reinforcing particles have properties that improve the properties of a pure elastomer. Such improved properties include increased thermal conductivity, modulus of stiffness, hardness, and velocity of sound. Improving such properties assists the nanocomposite layer in absorbing and dissipating the impact energy of the particulate matter over a larger volume in comparison to a pure elastomeric material. Specifically, because the size of sand is typically about 20 to 30 microns and the size of the water droplets is typically about 2000 microns, the size of the particulate matter is about 200 to 400,000 times larger than the size of the nanosized reinforcing particles. Hence, there is significant number of reinforcing particles per unit volume. In comparison to a pure, unreinforced elastomer, the nanocomposite layer is able to dissipate the particulate matter""s impact energy over a greater volume. Distributing the impact energy over a larger volume, thereby, reduces the energy density of the nanocomposite layer. The nanocomposite layer is, therefore, capable of withstanding the repeated impact caused by the particulate matter for a greater length of time compared to a pure elastomer. Because the life expectancy of a nanocomposite layer is greater than that of an elastomer, incorporating the present invention into a tape or coating would reduce the frequency at which it would need to be replaced.
Incorporating nanosized reinforcement particles into an elastomeric matrix also creates a more impact resistant layer than incorporating conventional reinforcement particles into such a matrix because conventional reinforcement particles are typically micron sized particles or fibers. Nanosized reinforcement particles are typically at least one hundred times smaller than conventional reinforcement particles. Incorporating nanosized reinforcement particles into an elastomeric matrix creates a more impact resistant layer than would incorporating conventional reinforcement particles or fibers into such a matrix because while the size of the impact area remains constant, the degree of reinforcement per unit volume of impact increases. In other words, utilizing nanosized reinforcing particles in lieu of conventional reinforcement particles significantly increases the number of reinforcing particles per unit volume. Thus, the impact energy density decreases. Hence, the nanocomposite layer""s increased reinforcement allows it to absorb and dissipate a greater amount of impact energy than a conventionally reinforced elastomer. The nanocomposite layer, therefore, is capable of withstanding the repeated impact caused by the particulate matter for a greater period compared to a conventionally reinforced elastomer.
Accordingly, the present invention relates to an airfoil that includes a nanocomposite layer affixed to an aerodynamic member, wherein the nanocomposite layer is an elastomeric material reinforced with nanometer sized reinforcing particles. The nanocomposite layer is applied to the aerodynamic member as either an adhesive tape or a coating, which is mechanically and/or chemically bonded to the aerodynamic member. If the nanocomposite layer is applied as a coating, the nanocomposite layer can be dip-coated, sprayed on or painted onto the aerodynamic member. Applying the nanocomposite layer either as a tape or spraying or painting it on the aerodynamic member are simple and relatively quick procedures such that the nanocomposite layer can be applied in the field, thereby improving the aircraft""s availability time.
Applying a nanocomposite layer over an aerodynamic member also increases the airfoil""s wear resistance. Hence the present invention increases the airfoil""s durability, thereby reducing the frequency at which the airfoil may require repair. Additionally, the nanocomposite layer is less expensive than an engineered ceramic component. The present invention, therefore, is a relatively inexpensive and simple xe2x80x9cfield levelxe2x80x9d repair technique that increases an airfoil""s wear resistance in an erosive environment such that the repair technique is required on an infrequent basis.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.